Rename _threadinfo to _cygtls, throughout.

Rename _threadinfo to _cygtls, throughout.
* cygtls.h (_cygtls::call_signal_handler): Rename from call_signal_handler_now.
(_cygtls::push): Make second argument mandatory.
(_cygtls::fixup_after_fork): Declare new function.
(_cygtls::lock): Ditto.
* cygtls.cc (_cygtls::fixup_after_fork): Define new function.
* dcrt0.cc (cygwin_finished_initializing): Define as bool.
(alloc_stack): Use _tlstop rather than arbitrary variable in probably vain
attempt to avoid strange fork problem on CTRL-C.
(dll_crt0_0): Remove obsolete winpids::init call.
* dll_init.cc (dll_dllcrt0): Detect forkee condition as equivalent to
initializing.
* winsup.h (cygwin_finished_initializing): Declare as bool.
* exceptions.cc (handle_exceptions): Rely on cygwin_finished_initializing to
determine how to handle exception during process startup.
(_cygtls::call_signal_handler): Rename from call_signal_handler_now.
(_cygtls::interrupt_now): Fill in second argument to push.
(signal_fixup_after_fork): Eliminate.
(setup_handler): Initialize locked to avoid potential inappropriate unlock.
Resume thread if it has acquired the stack lock.
(ctrl_c_handler): Just exit if ctrl-c is hit before cygiwn has finished
initializing.
* fork.cc (sync_with_child): Don't call abort since it can cause exit
deadlocks.
(sync_with_child): Change debugging output slightly.
(fork_child): Set cygwin_finished_initializing here.  Call _cygtls fork fixup
and explicitly call sigproc_init.
(fork_parent): Release malloc lock on fork failure.
(vfork): Call signal handler via _my_tls.
* sigproc.cc (sig_send): Ditto.
* syscalls.cc (readv): Ditto.
* termios.cc (tcsetattr): Ditto.
* wait.cc (wait4): Ditto.
* signal.cc (nanosleep): Ditto.
(abort): Ditto.
(kill_pgrp): Avoid killing self if exiting.
* sync.cc (muto::acquire): Remove (temporarily?) ill-advised exiting_thread
check.
* gendef (_sigfe): Be more agressive in protecting stack pointer from other
access by signal thread.
(_cygtls::locked): Define new function.
(_sigbe): Ditto.
(_cygtls::pop): Protect edx.
(_cygtls::lock): Use guaranteed method to set eax to 1.
(longjmp): Aggressively protect signal stack.
* miscfuncs.cc (low_priority_sleep): Reduce "sleep time" for secs == 0.
* pinfo.cc (winpids::set): Counterintuitively use malloc's lock to protect
simultaneous access to the pids list since there are pathological conditions
which can cause malloc to call winpid.
(winpids::init): Eliminate.
* pinfo.h (winpids::cs): Eliminate declaration.
* pinfo.h (winpids::init): Eliminate definition.